Many articles of commerce are made of zinc and are generally produced as zinc die castings or formed from sheet using rolled zinc alloys. To enhance the decorative features of articles made from zinc alloys or to improve their corrosion resistant properties, it is common practice to coat the zinc part with other metals. This is normally accomplished through electroplating of other metals onto the surface of the zinc article. The metals most commonly electrodeposited onto zinc surfaces to achieve these ends are copper, nickel, chromium, tin or brass.
Because of the relative anodic position of zinc in the electromotive series, most metals, with the exception of copper, cannot practically be electroplated directly over zinc. The usual practice in the art is to first deposit the copper onto a zinc surface and followed, if desired, by plating other metals (e.g., nickel, chromium, tin, etc.) over the copper. The electrodeposition of copper onto zinc surfaces is normally accomplished by using alkaline copper plating baths based on copper cyanide or pyrophosphates.
The fidelity of a finished plating involving one metal over another depends on many variables. One of the most important factors being the condition of the interface between the plated metal and its substrate. For example, if the surface of the substrate is not properly prepared or cleaned prior to plating, then poor adhesion or incomplete coverage of the plating can occur. This is true not only for plating copper over zinc but for any plating system and is emphasized to indicate the importance of the interface on the quality and stability of a plated system. However, with the plating of copper over zinc alloys another factor which can effect the stability or longer term fidelity of a given product and must be considered is related to the occurrence of interdiffusion of the copper plate and the zinc substrate.
It is known that when copper and zinc are placed in intimate contact, as is the case in electroplating, that both metals tend to diffuse into each other and to give raise to a diffusion layer. For example, one study by W. O. Allread and reported in Plating 49, 46 (1962) describes substantial diffusion taking place between a copper electroplate and a zinc die cast substrate occuring at 350.degree.-365.degree. F., and in relatively short times. Diffusion also occurs at room temperature but at a slower rate. It is also indicated by Allread that the resultant diffusion zone or layer consists of a brittle brass alloy and can be a potential source of spalling failure of the plating.
It can be appreciated that the thickness of the diffusion layer formed between a copper electroplate and a zinc substrate is dependent on the time and temperature the different metals are in contact after plating. In fact, with enough time and temperature exposure all of the copper of the original plating would be consumed or absorbed through the diffusion process. When this occurs the zinc substrate will "show-through" the copper plate effecting not only the appearance of the plated article but also its corrosion properties. If other metals (e.g., nickel, chromium) are plated over the copper, spalling or flaking of the plated part generally also occurs which in turn effects both its appearance and corrosion properties.
In order to assure the fidelity of a plated zinc part through its intended life, it is important to provide enough copper plate thickness to more than accommodate expected diffusion throughout the life of the part. To establish copper plate thickness requirements it is necessary to identify the variables which can effect the interdiffusion rates of copper plated zinc article. Expected temperature exposure of the plated article is an important variable effecting diffusion rates and must be considered in determining copper plate thickness requirements. However, other factors of a plating system can also effect the interdiffusion rates, and one of these, namely the composition of the zinc alloy, is the subject of the current invention.
If temperature were the only variable effecting the rates of diffusion in a copper-plated zinc part then providing enough copper plate thickness would be the only viable method of assuring fidelity of the plated part throughout its intended life. However, it has been shown that, independent of temperature, other variables related to the plated article can also effect the interdiffusion rates. For example, in studies carried out by H. J. Read and W. P. Minnear Plating, Febuary 1970, pp. 153-154 and Plating, April 1972, pp. 309-315, they have shown that surface preparation methods and/or alloy composition of the zinc substrate can also effect diffusion rates. It was further pointed out by these authors that the more heavily alloyed (e.g., 4% aluminum) zinc die castings exhibited a slower interdiffusion rate as compared to pure zinc and typical rolled zinc alloys. In addition, it was also shown that small amounts of alloying constituents typically used in making rolled zinc alloys have little or no effect on the diffusion rates of a copper plated zinc part. It was also suggested in these studies that the aluminum used in the zinc die castings decreased the diffusion rates and the more aluminum used the less the interdiffusion.
In recent years considerable effort has been directed to improving various properties of zinc base alloys. For instance, U.S. Pat. No. 2,102,869 discloses improving the tensile strength, impact strength, machinability and resistance to corrosion of extruded zinc alloys by providing a zinc alloy containing about 10 to about 15 percent aluminum, from about 1.5 to about 4 percent copper and from about 0.01 to about 0.04 percent magnesium, the balance being zinc.
U.S. Pat. No. 2,169,441 discloses improving the workability of a zinc base alloy by providing one containing from about 10 to about 15 percent aluminum, about 1.5 to about 4 percent copper, about 0.01 to about 0.04 percent magnesium, the balance being zinc having a purity of at least 99.98 percent.
U.S. Pat. No. 2,195,566 discloses a photoengraving zinc base alloy plate composition containing from about 0.05 percent to 0.4 percent of aluminum, about 0.0004 percent to 0.009 percent of magnesium and the balance high grade zinc. Aluminum is used in the alloy composition to improve the etching characteristics of the plate.
In U.S. Pat. No. 3,037,859 there is disclosed a zinc base alloy having improved mechanical properties containing from about 0.5 to about 5.0 percent aluminum, about 5 to about 10 percent copper, about 0.005 to about 0.30 percent magnesium, about 0.001 to about 0.30 percent beryllium, the balance being pure zinc.
A zinc base alloy having good machinability, improved corrosion properties and improved mechanical properties is disclosed in U.S. Pat. No. 3,676,115, said alloy containing about 70 to about 82 percent zinc, about 18 to about 30 percent aluminum, about 0.05 to about 0.25 percent magnesium and up to about 2 percent of copper, nickel or silver.
A zinc base alloy forging having improved strength, dimensional stability and impact resistance properties at low temperatures is disclosed in U.S. Pat. No. 3,734,785, the said alloy consisting essentially of about 9 to about 22 percent aluminum, about 0.5 to about 1.5 percent copper, about 0.01 to about 0.03 percent magnesium, the balance being zinc.
U.S. Pat. No. 3,790,373 discloses an aluminum-zinc alloy consisting essentially of about 38 to about 75 percent, 0 to about 0.05 percent magnesium, 0 to about 4.8 percent copper, about 0.05 to about 2.8 percent nickel and the balance being zinc.
Another zinc-aluminum alloy, disclosed as having good machinability and satisfactory corrosion resistance, is described in U.S. Pat. No. 3,798,028, the said alloy comprising about 18 to about 30 percent aluminum, about 0.01 to about 1 percent magnesium, about 0.01 to about 3 percent bismuth, 0 to about 5 percent copper, the balance being zinc.
Still another zinc-aluminum alloy is disclosed in U.S. Pat. No. 3,850,622 which contains about 20 to about 24 percent aluminum, about 0.75 to about 1.1 percent copper, about 0.04 to about 0.05 percent magnesium, about 0.01 to about 0.03 percent of at least one of calcium, lithium and sodium, the balance being zinc.
In U.S. Pat. No. 4,126,450 there is disclosed a castable zinc base alloy described as having highly favorable castability, tensile strength, tensile strength stability, shear strength and platability characteristics. The subject alloy comprises about 4 to about 10 percent aluminum, about 1 to about 6 percent copper, about 0.02 to about 0.04 percent magnesium, the balance being zinc.
U.S. Pat. No. 4,095,014 describes an article of manufacture made from a zinc base metal having a substantially continuous, wear-resistant hard chromium skin layer on at least one surface thereof. The zinc alloys disclosed therein have the following compositions: (1) zinc alloyed with 4 percent aluminum, 0.04 percent magnesium, a maximum of 0.25 percent copper, less than 0.1 percent iron, less than 0.005 percent lead, less than 0.004 percent cadmium and less than 0.003 percent tin; (2) zinc alloyed as in (1) but having 0.75-1.25 percent copper; and (3) an alloy comprising 95 percent zinc, 1.25 percent copper, 3.5 percent aluminum, 0.1 percent iron, 0.02 percent magnesium, 0.005 percent lead, 0.004 percent cadmium and 0.003 percent tin.
In light of the published information on the effects of alloy composition of zinc alloys on interdiffusion rates with copper plate, it was surprising to find that very small additions of aluminum to zinc alloy, especially rolled zinc alloys, had such a profound effect on slowing down the diffusion rates. The achievement of much slower diffusion rates through the addition of very small amounts of aluminum was unexpected and significant. It is particularly significant for rolled zinc alloys where it is important to minimize alloy additions consistent with the achievement of required mechanical and/or chemical properties of the rolled zinc.
While this development finds wide spread applicability, it is particularly advantageous in the field of producing blanks for minting into coins or similar disc-shaped articles, as well as other applications where rolled zinc alloy parts are initially plated with copper then optionally over plated with, for instance, nickel and/or chromium.